In an information society, a plurality of contents provider provides very large amount of information at the moment. According to development of communication technology and gradually extended diffusion of systems quickly processing a large amount of information, information transmission between systems by using a satellite is already applied in many fields. Particularly, the requirement of developing a more effective satellite antenna mounted on a mobile body is increased for smooth communication between a satellite and a satellite antenna mounted on mobile body such as vessels or vehicles.
A satellite antenna mounted on a mobile body is an apparatus for favorably receiving a satellite signal by precisely directing an antenna to a satellite when the mobile body is quickly moved or turned. Accordingly, a more delicate control technique to always track a satellite is required differently from a fixed satellite antenna.
A step tracking method is generally used as a satellite tracking method of a satellite antenna, in which a reflector is tilted in at least one specific direction, a satellite signal size in each position in which the reflector is tilted is measured, and the satellite signal sizes are compared with each other, thereby compensating the directional direction of the reflector. Since the step tracking method has a long sampling period in which a satellite signal is sampled in a direction in which the reflector is tilted in order to compensate the directional direction of the reflector, it is not easy that the directional direction of the reflector is quickly compensated in response to rapid movements of a mobile body.
In order to solve the problem caused by the size of the sampling period, when a mobile body is suddenly moved, the directional direction of the reflector in response to the movements of the mobile body is compensated by using a position sensor such as a gyro sensor having quick response speed. The directional direction of the reflector is fine tuned by tilting the reflector.
FIG. 1 is a diagram illustrating an error signal between a main lobe of a satellite signal and a directional direction of a satellite tracking antenna when a mobile body equipped with a satellite tracking antenna system according to a conventional art.
Referring to FIG. 1, when a mobile body equipped with a satellite tracking antenna is not moved, the satellite tracking antenna computes error information between a directional direction of a reflector and a position of a main lobe of a satellite signal by using a satellite signal level value
  v  ⁡      (          t      n      2        )  and a satellite signal level value
      v    ⁡          (              t        n        4            )        ,thereby generating an accurate error signal. The satellite signal level value
  v  ⁡      (          t      n      2        )  is a value of a state in which the reflector is tilted left, and the satellite signal level value
  v  ⁡      (          t      n      4        )  is a value of a state in which the reflector is tilted right. In this case, an equation
      e    azimuth    =            v      ⁡              (                  t          n          2                )              -          v      ⁡              (                  t          n          4                )            is for an azimuth error signal, and an equation
      e    elevation    =            v      ⁡              (                  t          n          1                )              -          v      ⁡              (                  t          n          3                )            is for an elevation error signal.
FIG. 2 is a diagram illustrating an error signal between a main lobe of a satellite signal and a directional direction of a satellite tracking antenna when a mobile body equipped with a satellite tracking antenna is moved left.
Referring to FIG. 2, a directional direction of a satellite tracking antenna is the same as shown in FIG. 1. When a mobile body equipped with a satellite tracking antenna is moved left caused by rotation or a sudden shaking, the satellite tracking antenna obtains a satellite signal level value
  v  ⁡      (          t      n      2        )  sampled in case that a reflector is tilted left and a satellite signal level value
  v  ⁡      (          t      n      4        )  sampled in case that the reflector is tilted right. In this case, an error occurs in a satellite signal level value with respect to a main lobe of a satellite signal caused by movement of the mobile body. Therefore, an error occurs in generating an error signal. Namely, since the satellite signal level value
  v  ⁡      (          t      n      2        )  sampled in case that the reflector is tilted left is obtained and the satellite signal level value
  v  ⁡      (          t      n      4        )  sampled at the position indicated by a dotted line of FIG. 2 in case that the reflector is tilted right caused by movement of the mobile body, an error signal larger than the original error signal. The described above may appear as the opposite phenomenon according to the order of obtaining satellite signal level values and generating error signals according to the direction of tilting the reflector.
FIG. 3 is a diagram illustrating an error signal between a main lobe of a satellite signal and a directional direction of a satellite tracking antenna when a mobile body equipped with a satellite tracking antenna is rotated right.
A distorted error signal of a value smaller than the original error signal is generated similar to the description of FIG. 2 when the mobile body of FIG. 3 is rotated right.
As illustrated in FIGS. 1 through 3, according to the satellite tracking antenna using the conventional step tracking, when a sudden movement of the mobile body equipped with the satellite tracking antenna occurs, an error is generated in generating an error signal with respect to a directional direction of a reflector, thereby not precisely tracking a satellite.
Also, there is a conical scan method as a satellite tracking method of a satellite tracking antenna, in which a sub-reflector of the satellite tracking antenna is tilted in at least one specific position, the size of a satellite signal sampled at each position in which the sub-reflector is tilted is measured, and the sizes of the satellite signals are compared with each other, thereby tracking a satellite.
Satellite tracking antenna tracking a satellite according to the conical scan method is widely used because the satellite tracking antenna tracks a satellite by using only satellite signals received from a satellite, thereby simply forming a system and not requiring compensation of a gyro sensor caused by an environmental factor.
However, a satellite tracking antenna employing a conventional conical scan method has a technical problem to set a sampling period which is information of the amount of time for precisely sampling a satellite signal at each position in which a sub-reflector is tilted. Also, when the sub-reflector is rotated and a satellite signal sampling period is determined according to the rotation period of the sub-reflector, the rotation period of the sub-reflector is continuously changed caused by aging and wear of a sub-reflector rotation part rotating the sub-reflector. When the rotation period of the sub-reflector is changed as described above, the satellite signal sampling period may be also changed. It is difficult to precisely track a satellite without frequently compensating the satellite signal sampling period.
As described above, according to a conventional satellite tracking antenna, since a satellite is tracked without compensating an error of a satellite tracking signal which may occur caused by the movement of a mobile body, a distorted result may be generated. Also, since a satellite is tracked by sampling a satellite signal without properly compensating a satellite signal sampling period according to rotation period change of a sub-reflector, tracking satellite is not precisely performed.
In order to solve the described problems, a satellite tracking antenna that can precisely and effectively track a satellite by compensating a directional direction of a reflector in response to the movement of a mobile body is required.